cannot be

[Ju]

inedible means cannot be eaten

How about cannot be :

1. heard
2. seen

Thank you.

[SirGod]

* Not a teacher

invisible, imperceptible, unseeable = cannot be seen
inaudible, imperceptible, silent = cannot be heard

There are more, but these seemed to be the ones that fit best. However, the choice depends on the context.

[BobK]

- But the 'synonyms' aren't always exact. A taste, noise, smell, imperfection, roughness in an otherwise smooth surface... they can all be 'imperceptible', for example. It just means 'not available to the organs of perception.

b